This invention relates to improved membranes for the separation of fluids made from polymers.
Permselective membranes for fluid separation are known and used commercially in applications such as the production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, separation of carbon dioxide from methane or nitrogen for the upgrading of natural gas streams, and the separation of hydrogen from various petrochemical and oil refining streams. Some membranes are made of materials that have high permeabilities, but exhibit low permselectivities. For certain fluid streams, one or more component or minor contaminant, such as organic solvents, may exhibit a strong interaction with the material of the membrane, which can result in the loss of performance due to plasticizing the membrane or other problems. Some membrane materials may offer resistance to this interaction with contaminants, but suffer from poor mechanical properties, resulting in membrane failure when exposed to high membrane differential pressures and high temperatures. Other materials, such as previously available polyimide polymers, are not capable of processing into membranes of the desired configuration (such as a hollow fiber membrane). A membrane with a good balance of high productivity and selectivity for the fluids of interest, and persistently good separation performance despite long-term contact with aggressive process composition, pressure and temperature conditions, and that can be processed into a wide variety of membrane configurations is highly desired.
Membranes of polyimide polymers are desirable for their chemical resistant properties. However, some commercially available polyimide polymers are low molecular weight (MW) and prone to hydrolysis. Solution spinning of these polymers results in brittle hollow fibers. Due to the poor mechanical properties of these fibers, the polyimide polymers are difficult to commercially use to produce gas separation membranes, particularly hollow fiber membranes.
The references discussed below describe separation membranes known in the art and disclose information relevant to polyimide polymer membranes. However, these references suffer from one or more of the disadvantages discussed above.
U.S. Pat. No. 4,705,540 discloses highly permeable polyimide gas separation membranes prepared from phenylene diamines having substituents on all positions ortho to the amine functions and a rigid dianhydride or mixtures thereof, specifically pyromellitic dianhydride (PMDA) and 4,4′-(hexafluoroisopropylidene)-bis(phthalic anhydride) (6FDA).
U.S. Pat. No. 4,717,393 shows that polyimides incorporating at least in part 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and phenylene diamines having substituents on all positions ortho to the amine functions can be photo chemically crosslinked. Photochemical crosslinking is not considered a practical method for fabricating cost-effective gas separation membranes.
U.S. Pat. No. 4,880,442 discloses highly permeable polyimide gas separation membranes prepared from phenylene diamines having substituents on all positions ortho to the amine functions and essentially nonrigid dianhydrides.
U.S. Pat. No. 5,055,116 and U.S. Pat. No. 5,635,067 describe blends of polyimides designed to attempt to create a membrane with desirable performance properties. Polymeric blending has traditionally been thought to be problematic or result in poor mechanical properties, and limited range of fluid transport properties.
U.S. Pat. Nos. 4,532,041, 4,571,444, 4,606,903, 4,836,927, 5,133,867, 6,180,008, and 6,187,987 disclose membranes based on a polyimide copolymer derived from the co-condensation of benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) and a mixture of di(4-aminophenyl)methane and a mixture of toluene diamines useful for liquid separations.
U.S. Pat. Nos. 5,605,627, 5,683,584, and 5,762,798 disclose asymmetric, microporous membranes based on a polyimide copolymer derived from the co-condensation of benzophenone-3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) and a mixture of di(4-aminophenyl)methane and a mixture of toluene diamines useful for liquid filtration or dialysis membranes.
It is highly desirable to create a membrane that can be used commercially in applications such as the production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, separation of carbon dioxide from methane or nitrogen for the upgrading of natural gas streams, and the separation of hydrogen from various petrochemical and oil refining streams. The desired membranes should exhibit a resistance to interaction of the material with the process and the resulting plasticizing of the membrane. Furthermore, membranes should have superior mechanical properties to allow the use of the membranes in high differential pressure applications, and should be capable of easily processing into membranes of the desired configuration (such as a hollow fiber membrane). Thus, membranes with a good balance of high productivity and selectivity for the fluids of interest, and persistently good separation performance despite long-term contact with aggressive process composition, pressure and temperature conditions are desired.